Secondary batteries are usually subject to contamination with metal impurities in the form of metal powder resulting from abrasion that occurs during their manufacture. Such metal contaminants electrochemically dissolve and disperse into the electrolytic solution under the influence of the electric potential of the positive electrode. The dissolved metal in the form of ion reaches the negative electrode and separates out there under the influence of the electric potential of the negative electrode. The metal which has separated out occasionally penetrates through the separator to bring about microshorts between the positive and negative electrodes.
There is a technology of preventing the deterioration of plates and the occurrence of microshorts by incorporating the electrolytic solution with sodium 1,3,6-naphthalene-trisulfonate or potassium thiocyanate as a leveling agent to suppress metal ion separation, as disclosed in Patent Document 1.    Patent Document 1: JP-2001-357874-A